1. Field of the Invention
The present invention relates in general to automatic transmissions for motor vehicles and more particularly to automatic transmissions of a type which are equipped with a one-way clutch friction suppression system. More specifically, the present invention is concerned with an improvement in the one-way clutch friction suppression system, which can provide the transmission with a fail-safe operation.
2. Description of the Prior Art
An automatic transmission comprises a plurality of friction elements, such as, friction clutches, friction brakes and the like, which are forced to engage and/or disengage by hydraulic power for obtaining various speed gear positions of the transmission.
Usually, in a lower speed gear position, the power transmission is carried out by using or engaging a one-way clutch. Upon requirement of up-shifting from such lower speed gear position to a higher speed gear position, the engaged condition of the one-way clutch is canceled or released. This method of up-shifting can reduce undesired select shock by a certain degree.
One of automatic transmissions of this type is disclosed in "MAINTENANCE MANUAL OF NISSAN VEHICLE" issued August, 1991 from NISSAN MOTOR CO., LTD. In this transmission, a forward one-way clutch is employed which corresponds to the above-mentioned one-way clutch. The one-way clutch becomes engaged to contribute to the power transmission when achieving first, second and third speed gear positions. Upon up-shifting from third to fourth speed gear position, the one-way clutch becomes disengaged and a band brake becomes engaged.
However, in the transmission disclosed in the publication, even in the fourth speed gear position, inner and outer races of the forward one-way clutch are kept rotated by respective rotating members, and thus, a certain friction is inevitably produced between the inner and outer races, which causes the power loss and thus deteriorates the fuel consumption of the engine.
In order to solve the above-mentioned drawback, a measure may be thought out in which a control valve is added to the hydraulic control circuit to drain the hydraulic pressure of a forward clutch to obtain a full released condition of the forward one-way clutch upon the up-shifting from third to fourth speed gear position. However, when employing this measure, a new fear may arise in that if, due to some accident, the control valve becomes stuck at such drain permitting position, engagement of the forward clutch is not obtained and thus first, second and third gear positions are not obtained any longer.